Dallas was the second of six children,[2] another of whom, Alexander, would become the commander of Pensacola Navy Yard. During Dallas's childhood, the family lived in a mansion on Fourth Street, with a second home in the countryside, situated on the Schuylkill River. He was educated privately at Quaker-run preparatory schools, before studying at the College of New Jersey (now Princeton University), from which he graduated with highest honors in 1810.[3] While at College, he participated in several activities, including the American Whig–Cliosophic Society.[4] Afterwards, he studied law, and was admitted to the Pennsylvania bar in 1813.[2]

Dallas did not have much enthusiasm at the time for legal practice, and wanted to fight in the War of 1812, a plan that he dropped due to his father's objection.[2] Just after this, Dallas accepted an offer to be the private secretary of Albert Gallatin, and he went to Russia with Gallatin who was sent there to try to secure its aid in peace negotiations between Great Britain and the United States.[2] Dallas enjoyed the opportunities offered to him by being in Russia, but after six months there he was ordered to go to London to determine whether the War of 1812 could be resolved diplomatically.[2] In August 1814, he arrived in Washington, D.C. and delivered a preliminary draft of Britain's peace terms.[2] There, he was appointed by James Madison to become the remitter of the treasury, which is considered a "convenient arrangement" because Dallas's father was serving at the time as that department's secretary.[2] Since the job did not entail a large workload, Dallas found time to develop his grasp of politics, his major vocational interest.[2] He later became the counsel to the Second Bank of the United States.[2] In 1817, Dallas's father died, ending Dallas's plan for a family law practice, and he stopped working for the Second Bank of the United States and became the deputy attorney general of Philadelphia, a position he held until 1820.[2]

After the War of 1812 ended, Pennsylvania's political climate was chaotic, with two factions in that state's Democratic party vying for control.[2] One, the Philadelphia-based "Family party", was led by Dallas, and it espoused the beliefs that the Constitution of the United States was supreme, that an energetic national government should exist that would implement protective tariffs, a powerful central banking system, and undertake internal improvements to the country in order to facilitate national commerce.[2] The other faction was called the "Amalgamators", headed by the future President James Buchanan.[2]

Voters elected Dallas Mayor of Philadelphia as the candidate of the Family party, after the party had gained control of the city councils.[2] However, he quickly grew bored of that post, and became the United States attorney for the eastern district of Pennsylvania in 1829, a position his father had held from 1801 to 1814, and continued in that role until 1831.[2] In December of that year, he won a five-man, eleven-ballot contest in the state legislature, that enabled him to become the Senator from Pennsylvania in order to complete the unexpired term[2] of the previous senator who had resigned.[5]

Dallas served less than fifteen months — from December 13, 1831, to March 4, 1833 — and declined to be a candidate for reelection. He was chairman of the Committee on Naval Affairs.

Polk/Dallas campaign poster.

Dallas resumed the practice of law, was attorney general of Pennsylvania from 1833 to 1835, and served as the Grand Master of Freemasons in Pennsylvania in 1835.[6] He was appointed by President Martin Van Buren as Envoy Extraordinary and Minister Plenipotentiary to Russia from 1837 to 1839, when he was recalled at his own request. Dallas was elected Vice President of the United States on the Democratic ticket in 1844 with James K. Polk and served from March 4, 1845 to March 4, 1849.

In 1837 President Van Buren appointed him minister to Russia, which post he retained till October, 1839, when he was recalled, at his own request, and again resumed legal practice. George M. Dallas and James Buchanan were for many years rival leaders of the Democratic Party in Pennsylvania, and aspirants for the presidency of the United States. In May, 1844, the democratic convention at Baltimore nominated him for vice-president of the United States on the ticket with James K. Polk for president. The democratic candidates were elected by an electoral vote of 170 out of 275. In 1856 Mr. Dallas succeeded Mr. Buchanan as minister to Great Britain, and continued in that post from 4 February 1856, until the appointment by President Lincoln of Charles F. Adams, who relieved him on 16 May 1861. At the very beginning of his diplomatic service in England he was called to act upon the Central American question, and the request made by the United States to the British government that Sir John Crampton, the British minister to the United States, should be recalled. Mr. Dallas managed these delicate questions in a conciliatory spirit, but without any sacrifice of national dignity, and both were settled amicably. At the close of his diplomatic career Mr. Dallas returned to private life and took no further part in public affairs except to express condemnation of secession.[7]

Dallas was a political rival with fellow Pennsylvanian James Buchanan, who would go on to be the 15th President of the United States. The rivalry was primarily centered around the power struggle in the Democratic party between the Dallas-led "Family party" and the rival "Amalgamators" led by James Buchanan. Intensity in the rivalry occurred during 1835, when "control of the state's party machinery shifted from the declining Family party to Buchanan's Amalgamators."[7] In 1845, Buchanan was appointed Secretary of State by President Polk, though not without Dallas's vehement objections.[8] Led by Dallas, the Philadelphia-based "Family party" shared his belief in the supremacy of the Constitution and in an active national government that would impose protective tariffs, operate a strong central banking system, and promote so-called internal improvements to facilitate national commerce. In factional opposition to Dallas stood the equally patrician James Buchanan of Harrisburg, head of the rival "Amalgamators," whose strength lay among the farmers of western Pennsylvania.

When the Family party gained control of the Philadelphia city councils, its members in 1828 elected Dallas as mayor. Boredom with that post quickly led Dallas—in his father's path—to the position of district attorney for the eastern district of Pennsylvania, where he stayed from 1829 to 1831. In December 1831 he won a five-man, eleven-ballot contest in the state legislature for election to the U.S. Senate to complete an unexpired term. In the Senate for only fourteen months, he chaired the Naval Affairs Committee and supported President Andrew Jackson's views on protective tariffs and the use of force to implement federal tariff laws in South Carolina.[7] Although off the national stage, Dallas remained active in state Democratic politics. The tension with Buchanan intensified when the latter returned from his diplomatic post in Russia and secured Pennsylvania's other seat in the U.S. Senate. Dallas turned down opportunities to return to the Senate and to become the nation's attorney general. Instead, he accepted an appointment as state attorney general, holding that post until 1835, when control of the state's party machinery shifted from the declining Family party to Buchanan's Amalgamators. In 1837, it was Dallas' turn for political exile, as newly elected President Martin Van Buren named him U.S. minister to Russia. Although Dallas enjoyed the social responsibilities of that post, he soon grew frustrated at its lack of substantive duties and returned to the United States in 1839. He found that during his absence in St. Petersburg Buchanan had achieved a commanding position in the home state political contest that had long engaged the two men.[7]

In December 1839, Van Buren offered the U.S. attorney-generalship to Dallas after Buchanan had rejected the post. Dallas again declined the offer and spent the following years building his Philadelphia law practice. His relations with Buchanan remained troubled throughout this period.

Dallas determined that he would use his vice-presidential position to advance two of the administration's major objectives: tariff reduction and territorial expansion. As a Pennsylvanian, Dallas had traditionally supported the protectionist tariff policy that his state's coal and iron interests demanded. But as vice president, elected on a platform dedicated to tariff reduction, he agreed to do anything necessary to realize that goal. Dallas equated the vice president's constitutional power to break tied votes in the Senate with the president's constitutional power to veto acts of Congress. At the end of his vice-presidential term, Dallas claimed that he cast thirty tie-breaking votes during his four years in office (although only nineteen of these have been identified in Senate records). Taking obvious personal satisfaction in this record, Dallas singled out this achievement and the fairness with which he believed he accomplished it in his farewell address to the Senate. Not interested in political suicide, however, Dallas sought to avoid having to exercise his singular constitutional prerogative on the tariff issue, actively lobbying senators during the debate over Treasury Secretary Walker's tariff bill in the summer of 1846. He complained to his wife (whom he sometimes addressed as "Mrs. Vice") that the Senate speeches on the subject were "as vapid as inexhaustible. . . . All sorts of ridiculous efforts are making, by letters, newspaper-paragraphs, and personal visits, to affect the Vice's casting vote, by persuasion or threat."

Despite Dallas' efforts to avoid taking a stand, the Senate completed its voting on the Walker Tariff with a 27-to-27 tie. (A twenty-eighth vote in favor was held in reserve by a senator who opposed the measure but agreed to follow the instructions of his state legislature to support it.) When he cast the tie-breaking vote in favor of the tariff on July 28, 1846, Dallas rationalized that he had studied the distribution of Senate support and concluded that backing for the measure came from all regions of the country. Additionally, the measure had overwhelmingly passed the House of Representatives, a body closer to public sentiment. He apprehensively explained to the citizens of Pennsylvania that "an officer, elected by the suffrages of all twenty-eight states, and bound by his oath and every constitutional obligation, faithfully and fairly to represent, in the execution of his high trust, all the citizens of the Union" could not "narrow his great sphere and act with reference only to [Pennsylvania's] interests." While his action, based on a mixture of party loyalty and political opportunism, earned Dallas the respect of the president and certain party leaders—and possible votes in 1848 from the southern and western states that supported low tariffs—it effectively demolished his home state political base, ending any serious prospects for future elective office. (He even advised his wife in a message hand-delivered by the Senate Sergeant at Arms, "If there be the slightest indication of a disposition to riot in the city of Philadelphia, owing to the passage of the Tariff Bill, pack up and bring the whole brood to Washington.")

While Dallas' tariff vote destroyed him in Pennsylvania, his aggressive views on Oregon and the Mexican War crippled his campaign efforts elsewhere in the nation. In his last hope of building the necessary national support to gain the White House, the vice president shifted his attention to the aggressive, expansionist foreign policy program embodied in the concept of "Manifest Destiny." He actively supported efforts to gain control of Texas, the Southwest, Cuba, and disputed portions of the Oregon territory.[7]